To meet the continued fast growing demand of mobile data, the wireless industry needs solutions that can provide very high data rates in a coverage area to multiple users simultaneously including at cell edges at reasonable cost. Currently, the wireless telecom industry is focused on dense deployment of small cells, the so called ultra-dense networks, to increase spatial re-use of wireless spectrum as the solution for meeting the growing mobile data demand. Dense deployment of small cells requires a large number of backhauls and creates highly complex inter-cell interference. One solution to the interference problem is to require careful Radio Frequency (RF) measurement and planning and inter-cell coordination, which significantly increases the cost of deployment and reduces the spectral efficiency. Another solution is the Self-Organizing Network (SON) technology, which senses the RF environments, configures the small cells accordingly through interference and transmitting management, coordinated transmission and handover. SON reduces the need for careful RF measurement and planning at the cost of increased management overhead and reduced spectral efficiency. The backhaul network to support a large number of small cells is expensive to be laid out. On Apr. 13, 2016, Facebook announced Terragraph and project ARIES (Antenna Radio Integration for Efficiency in Spectrum) as a terrestrial connectivity system as described in [1]. Terragraph is a 60 GHz, multi-node wireless system as a fiber replacement to bring high-speed Internet connectivity to dense urban areas. A distribution node is a 60 GHZ repeater that provides backhaul link that carries digital data to and from Wi-Fi (Wireless-Fidelity) or Ethernet access points, small cells, or other distribution node. It is fundamentally different from the wireless active antennas that use multiple layers of spatial multiplexing beamforming in two or more frequency bands and shifting up or down carrier frequencies.
Another method for increasing spatial re-use of wireless spectrum is Multiple-Input and Multiple-Output (MIMO), especially Multi-User MIMO (MU-MIMO). In a wireless communication system, a wireless node with multiple antennas, a network access point or Base Station (all referred to BS hereafter) or a User Equipment (UE), can use beamforming in the Downlink (DL) or the Uplink (UL) to increase the Signal-to-Noise Ratio (SNR) or Signal-to-Interference-plus-Noise Ratio (SINR), hence the data rate, of the links with other wireless nodes. MU-MIMO can beamform to multiple UEs simultaneously in a frequency and time block, e.g., a Resource Block (RB), i.e., using spatial multiplexing to provide capacity growth without the need of increasing the bandwidth. In a large-scale MIMO or massive MIMO system, a BS may be equipped with many tens to hundreds of antennas to further increase the gain from MIMO and the number of spatially multiplexed data streams. Although a MIMO BS with a large number of antennas can extend its DL coverage range through beamforming, the SINR of UEs can decay quickly as the distance between the BS and an UE increases, because UEs far away from the BS have significantly lower SINRs than UEs close to the BS due to large-scale fading, shadowing, and other factors. In addition, the UL range, and hence the UL channel estimation accuracy, is limited by the transmitting power of UEs. Before the BS knows the channels of the UEs, it is unable to perform beamforming.
To improve the coverage of a MIMO system, there are prior art that distributes the antennas of the BS and the associated RF components (referred to as Radio Units RUs, or Remote Radio Units RRUs) to the coverage area while keeping the MIMO beamforming computation at a central BS to reduce or eliminate the interference among the multiplexed beams. The connection between the BS and a RH is referred to as a fronthaul versus a backhaul that provides user data connection between a BS and an upper layer data server or controller or a core network. One way is to use an optical cable or Line-of-Sight (LoS) wireless link for the fronthaul that carries IQ samples such as in [2] which proposed a “network architecture with transceivers distributed serendipitously without any concept of a cell, exploiting high densification with fixed transmit power to increase spatial multiplexing gain. The transceivers are connected through a fronthaul and cooperate on a large scale to create concurrent spatial channels to multiple users via precoding.” As shown in FIG. 5 of [2], the fronthaul connections use fiber, gigabit Ethernet, LoS radio link or coax cable to connect to their pWave radio heads. As stated in [2], “The data center provides I/Q waveforms through fiber connections to RRHs called pWave radios, which consist only of analog-to-digital (A/D), digital-to-analog (D/A), and RF up/down converters, power amplifier and antenna”, and “The pCell processing then converts the U streams of user DL samples into N streams of pWave I/Q samples, which are finally transported to the pWave radios. The pWaves convert the I/Q samples to the RF domain and synchronously transmit the waveforms.” However, it is well known that the bandwidth required for I/Q (In-phase/Quadrature) samples in such a fronthaul link is several times higher than a backhaul, typically between 6 to 10 times higher. Moreover, the sampling I/Q data rate on the fronthaul is dependent on the number of antennas in the MIMO system and increases as the number of antennas increases in a MIMO network, as pointed out in [3]. This means that such a fronthaul solution requires very high bandwidth fiber connections, or if a wireless link is used to provide the fronthaul connection to transport the I/Q samples, it would require significantly higher wireless spectrum bandwidth than the wireless spectrum needed for the wireless link between a pCell and one or more UEs or more generally between a BS or a RRU and one or more UEs. This is a significant disadvantage and limitation of such a fronthaul based network, especially in the wireless fronthaul case since wireless spectra are scarce resources and higher spectral efficiency is highly preferred.
Prior art RF repeaters suffer the serious shortcomings of amplifying and introducing interference, thus often worsen the network performance. Power gain from a low gain prior art repeater disappears in a few meters if there is a LoS link between the BS and UE, and high gain prior art repeater creates interference to UEs that already have good SNR without the repeaters and worsens their performance. As a result, they were not favored by network operators and their use was limited.
Our PCT application PCT/US14/65853 entitled “Massive MIMO Multi-User Beamforming and Single Channel Full Duplex for Wireless Networks” presented inventions that use a massive MIMO BS to provide backhauls to distributed small cells, which can be generalized to distributed RRUs. Our PCT application PCT/US16/13742 entitled “Beamforming in a MU-MIMO Wireless Communication System With Relays” presented inventions that use a massive MIMO BS to beamform to UEs through distributed amplify-and-forward repeaters (referred to as AFRs) where the wireless connection between the BS and AFRs can use the same bandwidth as the wireless connection between the BS and UEs or between an AFR and UEs. However, in both PCT applications, the distributed RRUs and AFRs are single unit devices that receive a wireless signal from a BS and transmit the signal to one or more UEs, or vice versa. The preferred usage scenario of both cases are at locations where a local SNR gap of the signals at the BS-side antenna(s) and the UE-side antenna(s) exists, where a local SNR gap is defined as the strength of the BS signal outside a local area being significantly higher than the strength of the BS signal inside the local area signal. The SNR gap overcomes the shortcomings of prior art RF repeaters. An example of a local SNR gap is the BS signal inside a building, where the signal at an outdoor position and/or orientation is significantly stronger than indoors, e.g., 20 dB or higher. In such a scenario, the BS-side antenna(s) are placed outdoor to receive a strong BS signal, which is then amplified, forwarded, and transmitted through antenna(s) indoors to improve indoor coverage. The inventions in this application can improve the effectiveness and expand the applicability of the distributed AFRs in areas where a local SNR gap does not exist.
Another prior art moves part of the physical layer to the RRUs to reduce the bit rates required on the fronthaul, as proposed in NGFI (Next Generation Fronthaul Interface) [3], as a case of Xhaul (Crosshaul) [4], the integrated fonthaul and backhaul. The key to NGFI is the selection of the appropriate function split between the Base Band Unit (BBU) and the RRU. However, the function split in the NGFI would increase the complexity of the RRU and more importantly make it very difficult, if not impossible, for the BS to perform MIMO beamforming computation using distributed RRUs/antennas jointly at the BS, or as phrased in [3] “Some physical-layer-coordinated technologies are difficult to be implemented”.
This invention presents embodiments that use new classes of wireless active antennas or Wireless Smart Antennas (WSA) that support multi-user beamforming using distributed antennas and improve the consistency of the coverage of a MIMO BS while avoiding the high bandwidth requirements of prior art fronthaul connections and the serious shortcomings of prior art RF repeaters.
Use of centimeter and millimeter wave (all referred to as mm-wave hereafter) spectrum is a major trend in the upcoming 5G (Fifth-Generation) wireless networks. Major advantages of mm-wave include strong directivity thus low inter-beam interference, and availability of large bandwidth. Major limitations of mm-wave radio links include highly dependent on LoS conditions, wherein LoS condition is difficult to maintain between a BS or RRU(s) and UEs handled by mobile users, and difficulty serving fast moving UEs. This invention includes embodiments that make better use of the advantages of mm-wave to enhance mobile network throughput and coverage while overcoming its limitations.